Inverters are electric devices which convert or transform direct (DC) voltage into alternating (AC) voltage, that is, a direct current into an alternating current. Inverters should therefore be classified with the group of power converters.
Inverters can be designed for the generation of single-phase or polyphase alternating current also referred to as rotary current, of which the rotation direction follows from the phase shift between the phases. Inverters are typically used when electric consumers, such as, for instance, a motor, requires AC voltage in order to operate, but only a DC voltage power source is available, such as, for instance, a high-voltage vehicle battery, or something similar.
Furthermore, inverters are used when direct current must be fed into an alternating or three-phase current network, in particular in photovoltaic systems that are connected to the power grid for the generation of electricity.
Such inverters are also used for the powering of electric motors with a high-voltage, and therefore with a high, internally stored, intermediate circuit energy, specifically in vehicles with an on-board voltage of more than 60 V. For this voltage range, the motor vehicle industry also uses the term of high-voltage applications (HV applications). Such an application is, for instance, the use of an inverter for an electronically powered refrigerant compressor in a vehicle.
In prior art, inverters are built with a so-called B6 bridge, consisting of six power semiconductors or power switches. In such a B6 bridge, two respective power switches, which are jointly also referred to as a half bridge, are respectively arranged per output phase of the AC output voltage in a cross arm between a DC input voltage line, the tap for the respective output phase being located between the two power semiconductors. Customarily, a first power semiconductor arranged between the tap or the charge and the positive DC input voltage line is referred to as high-side power semiconductor or high-side switch, and the second power semiconductor, arranged between the tap and a negative DC input voltage line or a ground line, is referred to as low-side power semiconductor or low-side switch.
In such a configuration, the power semiconductors, which might, for instance, be a MOSFET, an IGBT, a tyristor, or similar, are controlled by a logic circuit to generate the desired output voltage. Since such logic circuits generally only provide low output voltages in the range of less than 5 V, it is customary to install a driver circuit between the output terminal of the logic circuit and the input terminals of the power semiconductors. This driver circuit provides an output voltage in the range of approximately 15 V, which is applied to the input terminals of the control electrodes of the power semiconductors.
In order to provide power to the driver circuit, which might, for instance, operate for the arranged electronic components at a voltage of 3 V or 5 V, and for providing a voltage of approximately 15 V to the power semiconductors, usually, a separate voltage supply must be provided.
Furthermore, according to prior art, the driver circuits and/or the power semiconductors in the half bridges are protected in high voltage inverters against an undesired parasitic activation by way of special protective measures.
It is therefore advantageous that the driver switch is constructed and operated such that a simultaneous activation of two power semiconductors in one half bridge is ruled out.
A further problem when operating an inverter is that especially in case of high voltage inverters, an undesired activation of the blocked power semiconductor may occur due to the high switch voltages and the steep increase of the switching pulse (dV/dt). The reason for such an undesired activation of the power semiconductor is the parasitic capacities in every real power semiconductor, which are also referred to as Miller capacities. Such a Miller capacity is known from the equivalent circuit diagram of a MOSFET or an IGBT.
As a result of a very fast voltage change in a very short time, a very high value for the dV/dt ratio of the switching voltage between HV+ and HV− is obtained, which might lead to a voltage on the control electrode of a blocked power semiconductor, which, if sufficiently large, may lead to the undesired activation of the power semiconductor.
In order to avoid such critical situations in the half bridge, it is known that the control electrode of the blocked power semiconductors can be switched against the ground potential.
Alternatively, it may be envisaged that the control electrode of the blocked power semiconductors is charged with a negative voltage (bias voltage). In that case, an additional negative bias voltage must be made available for the driver switch. This negative bias voltage may typically have a value of −5 V.
The provision of such a negative bias voltage, for instance for three low-side switches of a three-phase inverter, is possible by means of a single negative voltage of approx. −5 V.
For the high-side switches, on the other hand, of a negative bias voltage for every high-side switch is necessary, since the voltage at the tap is subject to a continuous change of its voltage value. This is also commonly referred to as a floating potential. The provision of such a negative bias voltage is usually accomplished by means of an additional secondary winding on a transformer, which is available anyway. For three negative bias voltages, therefore, three additional secondary windings on the transformer must be provided. Specifically for high-performance inverters that operate in a voltage range of 800 V to approx. 1000 V, appropriate insulation layers must be provided between the individual windings on the transformer core. In addition to the additionally required winding material, this leads to an increase of the construction size and of the weight of the transformer. Moreover, the production cost of such assemblies is higher.
Furthermore, when the transformer is installed on the surface of a circuit board on which the connections of the transformer are soldered to conducting paths on that surface, the forces operating on these conducting paths and on the solder joints increase. This increase of the operating forces is related to the increase in weight. This fact is particularly critical in assemblies that are intended for applications in vehicles, since in vehicles, vibrations occur which lead to an increased load on the soldering joints. This can only be remedied by means of mechanical fixation, for instance by means of adhesion or brackets, involving additional costs and expenses.